Many people have fled from Beijing

Large numbers of Chinese migrant workers have ignored government demands to remain in Sars-hit cities and returned to their rural homes, according to reports.

A million migrant workers are said to have arrived in the rural province of Anhui in recent weeks, with a further 800,000 returning to the neighbouring province of Henan, raising the prospect that some may have carried the virus with them.

Chinese Premier Wen Jiabao has warned of dire consequences if the deadly Sars virus spread to rural China, where 70% of the country's 1.3 billion people live.

"We must be on high alert," said Mr Wen, repeating a plea for migrant workers living in cities such as Beijing and Shanghai not to return home.

But a BBC correspondent in the capital, Rupert Wingfield-Hayes, says the warning may come too late.

The China News Service acknowledged on Wednesday that it was "very difficult to advise migrant workers in affected areas not to return home."

Part of the problem is that China has so many migrant workers.

An estimated 100 million Chinese have moved from the countryside to the cities in the last 15 years as the country's economic reforms gathered pace.

The booming growth of China's cities plus the perennial hard life in rural areas has led to this mass exodus. City employers have been quick to make use of the cheap labour pouring off trains and buses every day.

Migrants are often employed on construction sites, in factories and catering establishments, or as domestic helpers.

Most of the migrants come from poorer, inland provinces like Henan and Anhui.

As of Wednesday, official figures suggest these areas have escaped the Sars outbreak. Henan had only reported 15 Sars cases, while Anhui had just nine.

70% of China's 1.3 billion people live in rural areas

These figures are in stark contrast to the capital Beijing, which has seen more than 2,000 cases of infection - nearly half the county's total - and 110 deaths.

There are fears that with migrants returning home from places like Beijing, the disease could already have spread into areas without the money and infrastructure to contain it.

Poor health system

Correspondents say the health system in rural China is poor at best.

On Wednesday, Wen Jiabao admitted that "the basic rural medical facilities are weak," according to the official Xinhua news agency.

For those migrants who have already returned, BBC correspondent Francis Markus says the best hope to prevent the spread of Sars is to get them and their families to seek prompt treatment if they fall ill.

The government has pledged that those who cannot afford Sars treatment will be given free care.

But according to health workers, many of China's peasant population may be concerned that if they turn out not to have the virus, they could be faced with large medical bills.

On Wednesday, Anhui's provincial governor acknowledged that some migrant workers had already returned home.

He said the authorities had been trying to make the returnees aware of the dangers of Sars, and of how to prevent it.